Non-aqueous secondary batteries (hereinafter, also referred to simply as “secondary batteries”), such as lithium ion secondary batteries, have characteristics such as compact size, light weight, high energy density, and the ability to be repeatedly charged and discharged, and are used in a wide variety of applications. A secondary battery generally includes battery members such as a positive electrode, a negative electrode, and a separator that isolates the positive electrode and the negative electrode from one another and prevents short-circuiting between the positive and negative electrodes.
Battery members that include functional layers for providing these battery members with desired performance (for example, heat resistance and strength) are used in secondary batteries. Specifically, a separator obtained by forming a functional layer on a separator substrate or an electrode obtained by forming a functional layer on an electrode substrate including an electrode mixed material layer on a current collector may, for example, be used as a battery member. Moreover, one example of a functional layer that can improve heat resistance, strength, and so forth of a battery member is a functional layer that is composed of a porous membrane layer formed through binding of non-conductive particles by a binder (binding material). Moreover, a functional layer may, for example, be disposed between battery members in order to adhere the battery members to one another. Such a functional layer is formed by, for example, applying a composition for a functional layer that contains non-conductive particles, a binder, and a dispersion medium onto a substrate (for example, a separator substrate or an electrode substrate), and then drying the applied composition for a functional layer.
In recent years, much effort has been focused on enhancing compositions for non-aqueous secondary battery functional layers used in the formation of functional layers with the aim of further raising non-aqueous secondary battery performance (for example, refer to PTL 1).
In one specific example, PTL 1 proposes a composition for a non-aqueous secondary battery functional layer containing a particulate polymer that is a random copolymer comprising 35 mass % or more of a (meth)acrylic acid alkyl ester monomer unit and at least 30 mass % and not more than 65 mass % of an aromatic monovinyl monomer unit, and that has a degree of swelling in non-aqueous electrolyte solution of more than a factor of 1 and not more than a factor of 2. The composition for a non-aqueous secondary battery functional layer of PTL 1 can form a porous membrane having excellent durability and has excellent stability under high-shear.